01 -
Preheat the oven to 350ºF. For thick brownies, spray a 9x9" pan with non-stick spray or line with parchment paper. For thinner brownies, use a 9x13" pan.
02 -
Prepare the brownie mix according to the package directions. Pour the batter into the prepared pan and bake for 30 minutes, or until almost done. Do not overcook.
03 -
Remove the brownies from the oven and sprinkle 2 cups mini marshmallows evenly over the top. Bake for an additional 3 minutes, or until the marshmallows have puffed up. Remove from the oven and allow to cool.
04 -
In a large bowl, combine 1/4 cup salted butter, 1 1/2 cups powdered sugar, 2 Tablespoons cocoa powder, 2 Tablespoons milk, and 1/2 teaspoon vanilla extract. Beat with an electric mixer until smooth. Add more milk or powdered sugar as needed to achieve a smooth and creamy consistency.
05 -
Place spoonfuls of the frosting on top of the cooled marshmallows. Gently spread it evenly across the top. Cut the brownies into 5x4 rows using a plastic knife.